The following relates to the diagnostic imaging arts. It finds particular application in non-invasive measurement by magnetic resonance imaging of cerebral blood volumes, and will be described with particular reference thereto. However, it also finds application in measurement by magnetic resonance imaging of blood volumes in other tissues.
In-situ measurement of blood volume is useful in various clinical, diagnostic, and research applications. Local cerebral blood volume changes, for example, correlate with local neuronal activity in the brain. Cerebral blood volume measurements during physiological stimulation thus provides a tool for functional studies of brain activity. Cerebral blood volume measurements can also provide information about impaired and/or damaged tissue in stroke victims, as well as about lesions in many disorders, including, but not limited to cancer, vascular disorders, and the like. Blood volume imaging of other organs besides the brain can similarly provide functional and diagnostic data that is useful in clinical studies, diagnoses, and tests (e.g. stress tests or tests of vascular compliance).
Various imaging modalities have been used to measure blood volume characteristics, including positron emission tomography, single photon emission computed tomography, and magnetic resonance imaging. Most of these are invasive approaches, in which the subject is administered a suitable contrast agent that selectively enhances blood contrast in the selected imaging modality. For magnetic resonance imaging, various paramagnetic contrast agents are commonly used for this purpose. The requirement of an administered contrast agent is a substantial disadvantage of these techniques.
Magnetic resonance imaging of blood oxygenation level dependence (BOLD) is a non-invasive technique for indirectly measuring blood volume. In this technique, blood hemoglobin is used as an endogenous contrast agent. In one BOLD imaging approach, magnetic resonance imaging is performed as a function of physiological stimulation that causes changes in blood oxygenation level. Blood volume is estimated from BOLD measurements by making assumptions pertaining to other parameters that affect blood oxygenation level, such as blood flow. Hence, BOLD does not provide a direct measure of the blood volume.
A disadvantage of both the invasive techniques and the BOLD techniques as applied to blood volume measurement is that these existing techniques generally do not differentiate between blood in large blood vessels, on the one hand, and perfused blood in small capillaries or other microvessels, on the other hand. The blood volume in larger blood vessels is principally controlled by sympathetic regulation. In contrast, blood volume in microvessels having typical diameters of less than about 200 microns tends to vary to maintain local homeostasis or in response to chemicals such as vasodilators or vasorestrictive compounds. Consequently, the blood volume of microvessels responds to physiological perturbations such as local neuronal activity. For functional magnetic resonance imaging, the volume of blood in the microvessels is typically of principle interest, while the blood signal from larger blood vessels is interfering and thus undesirable. On the other hand, total microvascular plus macrovascular blood volume may change in some diseases, including but not limited to for instance arteriovenous malformations.
The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.